WEATHER 169 



driving vessels ashore and stripping plantations of every 

 vestige of usable plant growth. 



Tornadoes differ from hurricanes and typhoons in 

 duration and area affected. They originate in the region 

 of the prevailing westerlies and are quite common in the 

 plains in the western Mississippi valley. Their diameter 

 varies from a few feet to a mile or even more, but the 

 violent, destructive effects are usually confined to a path 



U. S. Weather Bureau. 

 FIG. 143. A Tornado Cloud as Seen from Pierre, South Dakota, in July, 1910. 



a few rods wide. In the summer season they are often 

 associated with thunderstorms. As they approach they 

 have the appearance of a funnel-shaped cloud hanging from 

 the black mass of thunder cloud above (Figure 143) . They 

 are the most violent storms which occur in the United 

 States. As a tornado approaches, a low roar which in- 

 creases momentarily in intensity is heard. Thunder, light- 

 ning, rain, and hail blend to make the effect more terrible. 

 Buildings that lie in the path of such a storm are scattered 



